The present invention generally relates to a method of forming local oxidation in a semiconductor device and more particularly, relates to a method of forming local oxidation having reduced bird""s beak encroachment in a semiconductor device.
In the manufacturing of semiconductor devices, local oxidation of silicon (also known as LOCOS) is a widely used processing step in forming lateral isolation between devices on a chip. The LOCOS process produces a fairly planar surface which is highly desirable for resolving and patterning dense features on an integrated circuit chip.
In a typical LOCOS process, a thin layer of pad oxide 12 of approximately 10 nm thickness is first formed on the surface of a silicon substrate 14. A pad oxide layer is a thin silicon oxide layer that is formed directly on silicon. This is shown in FIG. 1A. A thicker layer of silicon nitride 16 of approximately 100 nm is then deposited on top of the oxide layer 12. A photoresist layer 18 is used to pattern the nitride layer 16 leaving nitride only over the active regions 20. Following a channel-stop implantation (not shown), the photoresist mask 18 is stripped, leaving the previously masked oxide/nitride layers 12, 16 defining the active regions 20. The silicon substrate 14 is then subjected to an oxidation cycle to grow a thick field oxide layer 22 of approximately 600 nm. This is shown in FIG. 1B. Even though the oxide grows mainly in areas where the silicon nitride layer is absent, it grows in both directions vertically and also laterally under the oxide/nitride layers 12, 16. The lateral growth results in a so-called xe2x80x9cbird""s beakxe2x80x9d encroachment because of the shape of the oxide grown under the isolation mask. Since the beak to beak distance over the substrate not covered by oxide is the active transistor area, the smaller the beak the tighter devices can be packed on a substrate. In other words, the bird""s beak encroachment leads to an active area that is narrower than originally patterned. When a large bird""s beak is formed, the width of a transistor is reduced from that originally designed by photolithography. The growth of oxide under the nitride layer further causes stress problems in the oxide isolation formed. It is therefore desirable to form an oxide isolation that has the smallest bird""s beak.
Other workers in the field of the LOCOS process have proposed various techniques to minimize the bird""s beak effect. For instance, techniques such as side-wall masked isolation (or SWAMI) and poly-buffer LOCOS (or PBLOCOS) have been developed to reduce the effect of the bird""s beak. Another modified LOCOS technique produces a 0.6 xcexcm P+-P31  spacing by both substituting an oxinitride stress relieving layer for the silicon dioxide stress relieving layer in a conventional LOCOS process and by executing a sacrificial oxidation step prior to the growth of the channel oxide layer.
Still others have attempted to improve the LOCOS process. For instance, U.S. Pat. No. 5,254,495 discloses a method in which a suicide recessing process is incorporated into the local oxidation process. The process requires the additional steps of depositing a metal layer overlying the silicon nitride layer into the opening of the substrate, ion implanting channel-stops into the substrate, forming suicide between metal and the silicon substrate, and removing the metal silicide regions. It is therefore a more complicated process that incurs additional processing time and costs.
It is therefore an object of the present invention to provide a method of forming a localized oxidation in a semiconductor substrate with reduced oxide encroachment that does not have the drawbacks and shortcomings of the prior art methods.
It is another object of the present invention to provide a method of forming a localized oxidation with reduced oxide encroachment without the need for additional processing steps.
It is a further object of the present invention to provide a method for localized oxidation with reduced bird""s beak encroachment that can be carried out in a conventional LOCOS process.
It is another further object of the present invention to provide a method for localized oxidation with reduced bird""s beak encroachment that is capable of reducing the encroachment length significantly.
It is yet another object of the present invention to provide a method for localized oxidation with reduced oxide encroachment that can be carried out by first providing a sloped silicon recess in the substrate prior to the oxide formation.
In accordance with the present invention, a method of forming local oxidation having minimum lateral oxide growth or bird""s beak encroachment is provided. The method also provides a local oxidation with reduced residual stress in the oxide.
In a preferred embodiment, the method of forming a local oxidation is carried out by first providing a sloped silicon recess in the silicon substrate during a recess etching process. While any method can be used to provide a sloped silicon recess to achieve the desirable result, a novel etchant gas mixture is used in the present invention method. The sloped sidewalls of the silicon recess in the substrate effectively move the bottom of the exposed silicon away from the patterning resist edge. When compared to non-sloped silicon sidewalls, the encroaching oxidation starts with a built-in offset from the patterning edge which leads to a reduction in encroachment. Another benefit achieved by the present invention method is the reduction of stress in the oxide formed. The method greatly reduces stress defects in silicon generally caused by aggressive nitride to pad oxide thickness ratio in an attempt to reduce bird""s beak.
In the preferred embodiment, the present invention method is carried out by first providing a silicon substrate, forming a pad oxide layer on top of the substrate, forming a dielectric layer on top of the pad oxide layer, depositing a photoresist layer that has openings over areas of oxide isolations to be formed on the dielectric material layer, removing the dielectric material, the silicon oxide, and the silicon in the openings by an etching process such that a tapered recess is formed in the silicon substrate, growing a field oxide region within the tapered recess in the silicon substrate, and removing the layers of dielectric material and silicon oxide from the silicon substrate. The desirable range of taper for the silicon sidewalls in the recess is between about 10xc2x0 and about 75xc2x0 as measured from the vertical axis of the recess opening.